The above-described basecoat/clearcoat process is employed in particular for producing multicoat paint systems, especially metallic finishes on car bodies (cf. e.g. U.S. Pat. No. 3,639,147 and EP-A-38 127).
Using the basecoat/clearcoat process it is possible to produce finishes which, in comparison to one-coat topcoat systems, feature an improved rendering of the effect and feature the possibility of producing paint systems with bright and cleaner shades.
The basecoat applied first in stage (1), depending on the nature, amount and spatial orientation of the pigments employed, determines the shade and, if appropriate, the effect (e.g. metallic or pearlescent effect) of the paint system.
In stage (2) of the process, in an evaporation phase, at least some of the organic solvents and/or of the water is removed from the basecoat film applied in stage (1). Atop this pre-dried but unbaked basecoat film there is applied, in stage (3), a nonaqueous transparent topcoat (wet-on-wet technique) and then, in stage (4), basecoat film and topcoat film are baked together.
The transparent topcoat applied in stage (3) imparts gloss and fullness to the two-coat paint system and protects the pigmented paint film applied in stage (1) against chemical and physical attack.
Using the process under discussion it is possible to obtain high-quality two-coat paint systems only when the transparent topcoat applied in stage (3) does not interfere with the basecoat applied in stages (1) and (2) in such a way that there is impairment of the optical effect (e.g. clouding). On the other hand, the composition of the transparent topcoat must be such that it adheres well to the basecoat film following the baking process conducted in stage (4). Further important properties which the transparent topcoat film obtained after the baking process must have are high transparency, good gloss and good mechanical properties, such as hardness, scratch resistance and elasticity. The transparent topcoat film obtained after the baking process is required not least to have high resistance to climatic effects (e.g. temperature fluctuations, moisture in the form of water vapor, rain, dew, attack by radiation etc.) and to attack by acids or other chemicals, such as organic solvents, for example.
Furthermore, the transparent topcoats applied in stage (3) should have a very low content of organic solvents, and good stability on storage.
JP-A-2-242867 describes a basecoat/clearcoat process in stage (3) of which nonaqueous transparent topcoats are applied which comprise (A) a hydroxyl-containing synthetic resin, (B) an amino resin and (C) a blocked polyisocyanate, where components (B) and (C) are to be selected such that the temperature at which a chemical reaction ensues between (A) and (C) is not more than 20.degree. C. below and not more than 50.degree. C. above the temperature at which a chemical reaction ensues between (A) and (B).
Blocking agents specified for preparing component (C) are: volatile compounds of low molecular mass containing active hydrogen atoms, such as methanol, ethanol, propanol, butanol, hexanol, cyclohexanol, benzyl alcohol, ethylene glycol monoethyl ether and other aliphatic or aromatic monoalcohols, dimethyl- or diethylaminoethanol and other hydroxyl-containing tertiary amines, acetone oxime, methyl ethyl ketone oxime and other oximes, acetylacetone, acetoacetates, malonates and other compounds containing active methylene groups, .epsilon.-caprolactam and other lactams, and phenol. Blocking agents preferably employed are aliphatic monoalcohols, oximes and caprolactams.
The transparent topcoats described in JP-A-2-242867 give rise to paint systems which, especially in terms of their resistance to organic solvents and acids, gloss, transparency and resistance to yellowing, are to be improved.
DE-B-26 39 491 describes nonaqueous coating materials which comprise a hydroxyl-containing polyester resin and/or alkyd resin, hexamethylene diisocyanate blocked with an alkyl acetoacetate and/or 2,2,4 trimethylhexamethylene [sic] diisocyanate blocked with an alkyl acetoacetate, and an amino resin. These coating materials can also be employed as transparent topcoats in the sector of automotive finishing. The finishes obtained with these coating systems, especially when heightened baking temperatures and/or prolonged baking times are employed, undergo yellowing and are to be improved especially in terms of their resistance to acids and organic solvents and in terms of their scratch resistance.
The properties of the coating materials can be improved by the use of specific crosslinking agents. Mention should be made here in particular of the tris(alkoxycarbonylamino)triazines and derivatives thereof that are known from U.S. Pat. Nos. 5,084,541, 4,939,213, 5,288,865, 4,710,542 and from the EP Applications 0565774, 0541966, 0604922 and EP-B 0245700. Coating materials of this kind feature an especially good chemical resistance, which is evident in particular in the good results of outdoor weathering in Jacksonville, Fla. The tight crosslinking of such formulations, however, has the disadvantage of increased sensitivity toward moisture stresses, which is manifested in blushing of the coating films after such stresses.